Methylphenidate, hydrochloride (CAS No. 298-59-9) will be administered in feed to male and female F344/N rats and B6C3F1 mice for 104 weeks, in order to determine the effects of chronic exposure. Interim tests (clinical chemistry and hematology, clinical observations, and histopathology) will be performed at 39 and 65 weeks of treatment. Clinical observations, body weights, feed consumption, and histopathology on animals dying early, will be performed routinely. This data will be used to determine: (1) target organs related to treatment, (2) effects of treatment on central nervous system, bone, weight gain, liver and kidney function, serum enzymes, hematopoiesis, (3) difference in sensitivity between species and sexes, and (4) carcinogenic potential.